Process for the concentration by evaporation of phosphoric acid



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Phosphate rock'such as is ordinarily used for the production of phosphoric acid by treatment with sulfuric acid usually contains fluorine, sodium, and potassium compounds b which appear in the crude phosphoric acid as sodium and potassium fiuosilicates and, since the quantity of sodium and potassium ordinarily present is insuficient to combine with allfofthe hydrofluosilicic acid derived from. 1 the rock, the crude phosphoric acid ordinarily contains some 'free hydrofluosilicic acid.

In the concentration of such crude phosphoric acid in apparatus made of lead andother metals and alloys available for that 5 purpose the apparatus isv attacked principally by the hydrofluosilicic acid and in time destroyed.

I have observed that in the concentration of such crude hosph'oric acid, as the concen-' tratlon' rocee' s, a point is'reachedat which It has occurred to me that this precipitation of alkali metal lluosilicates with the for- 85 mation of crusts upon the concentrating apparatus during t advantage not only for the recovery of the alkali metal fiuosllicates as a valuable byurification of the crudephosphoric acid, at also for the protection of the concentrating apparatus against corrosion by'the crusts formed thereon. v

.A practically complete recove tluosilicate content of the crude acid with a simultaneous purification of the acid may be accomplished by supplying to the crude acid either before or during its concentration a 0 quantity of a soluble alkali metal compound,-

such as sodium carbonate, sufiic'ient to convert the hydrofluosilicic. acid content of the crude phosphoric acid into the corresponding alkali metal fluosilicates. lncidentally,

6 due neutralization of the hydrofluo- Vformed as quickly as paratus.

the concentrating apparatus and ingthe corrosion of the apparatus until the e concentration of the crude phosphoric acidmight beturned to v apparatus may be supplied initially with an of the to combine with the free by 1' Application filed January 30,- 1826. Serial No. 85,022.

silicic acid, its corrosive action upon the con centrating apparatus is largely or entirely eliminated. I v

For the full utilization in a practical way ofthe; protective action of the precipitated alkali metal fiuosilicate crusts several factors must be taken into consideration. In the firstplace the protective crustsshould be possible after the concentrating apparatus is put into use. Then theprotective crusts should be continuously maintainedythey should'be firmly adherent and continuous or "impervious; and they should-be maintainedof such thickness as to provide the desired protection to the concentrating apparatus without, however, being allowed to become so thick as to clog the apparatus or seriously interfere with the heat transfer involved in the operation of the apw The precipitation of firmly adherent coatings of the alkali metal fluosilicates upon the.

concentrating apparatus may be accom- 'plished in accordance with my invention by maintaining in the concentrating apparatus or evaporator a bath of acid, the concentration' of which with respect to alkali metal fluosilicates is at or above the saturationpoint as the concentration of the acid proceeds until the desired protective coating is formed. In starting a new concentrating apparatus it may be simply supplied with crude phosphoric acid and the concentration proceeded with in the usual way disregardpoint of saturation with alkali metal fluosilicates is. reached and the protective crust of alkali metal fiuosilicate is formed or the acid already saturated with alkali metal fluosilicate, so that a protective crust is immediately formed. The protective crust may then bebuilt up to the-desired thickness by the continued operation of the evaporating apparatus, either by withdrawing the concentrated acid and supplying fresh crude acid (batch operation) or by continuously supplying fresh crude acid at such a rate as to maintain the contents of the evaporator at the saturation point with respect to alkali fluosilicates, the latter method being preferred.

e addition of a soluble alkali metal coma pound to the crude acid in uantity suficient ofluosilieic acid while; resulting in the recovery of alkali metal fiuosilicates as by-products and in the purification of the crude phosphoric acid and assisting the formation of the protective crusts and reducing the corrosive action of the crude acid by the neutralization of the free hydrofluosilicic acid is not essential to the formation of the protective alkali metal fluosilicate crusts. The protective coating of till alkali metal fluosilicate may be formed from the normal alkali metal 'fiuosilicate content of the crude phosphoric acid, although as is apparent the addition of sufiicient alkali metal to the crude acid to combine with the free hydrofluosilicic acid is a preferred proce'dure,-

It is noted that in adding a soluble alkali metal compound to the crude phosphoric acid the quantity so added should not be sufiicient to precipitate the alkali metal fluosilicates prior to the concentration of the acid. As is known, alkali metal fiuosilicates are precipitated from crude phosphoric acid solutions without concentration by the addition of a soluble alkali metal compound such as sodium carbonate in quantity suflicient to combine with the hydrofluosilicicacid and to convert a portion of the phosphoric acid 'content of the solution, say, about per cent of it, into the mono alkali metal phosphate. Thequantity of soluble alkali metal compound added to the crude phosphoric acid in accordance with my invention therefore should be just suiiicient to convert the hydrofluosilicic acid content. of the solution into alkali metal fiuosilicate without precipitating it, With crude phosphoric acid of the usual strength, say 1.2 specific gravity, the quantity of soluble alkali metal compound added may be just suflicient or only slightly in excess of the quantity necessary to combine with the free hydrofluosilicic acid in the solution,

After the alkali metal fiuosilicate coating .has been formed uponthe concentrating vapparatus as described, and the desired thickness has been. reached, the thicknessis regulated hyan occasional treatment with water or sulfuric acid. That is, as the apparatus is used for the concentration of more crude phosphoric acid the coating becomes thicker and thicker, and in order to prevent it from becoming so thick as to interfere with the heat transfer of the apparatus or to 010 the apparatus, the concentration of aci is stopped from time to time and the apparatus filled with boiling water or sulfuric. acid which dissolves the coating, this treatment being continued until the coating is reduced to the desired thickness.=

This process which has been worked out in detail for the production of protective coatings upon apparatus used for the concentration' of phosphoric acid ap arently is applicable in other relationsjan in the treatment of other'liquids capable-of depositing adherent protective; crusts. I

I claim: v 1. In the concentration by evaporation of crude phosphoric acid solutions containing fluorides the step of protecting the concentrating apparatus against corrosion, which consists 1n depositing upon the concentrating apparatus protective crusts of alkali metal fluosilicates by maintaining in the concentrating apparatus during the concentrating operation a solution of hosphoric acid saturated with respect to al ali metal fluosilicates.

2. In the concentration by evaporation of crude phosphoric acid solutions containing fluorides the steps which consist in depositing protective crusts of alkali metal fiuo silicates u on the concentrating apparatus I which comprises adding to said phosphoric acid solution an alkali metal compound solu hle therein in quantity suflicient .to combine with said hydI'OfiUOSIllOiC' acid butv insuficient to precipitate alkali metal fiuosilicate, I

and subjecting the resulting phosphoric acid solution containing alkali metal fluosilicate to concentration by evaporation, whereby protective crusts of alkali metal fluosilicates are formed upon the conoenti'atingapparatus.

In testimony whereof, I aflixniy signature,

ELDON L. LARISON. 

